1. Field of the Invention
The present invention relates to a press molding method for a lens, particularly to a molding method for a lens having excellent shape and surface accuracy.
2. Description of the Prior Art
A conventional press molding method for a glass lens will be explained with reference to FIGS. 4-7. In a case of making a glass lens by press molding, generally a lens material is cut into a specified size, preheated to a temperature near the glass transition point, placed between upper and lower molds which are formed so that the cavity, when closed, forms the same shape as that of a finished lens, and press-molded under a specified temperature and a specified pressure.
The shape of the lens material 1 is preferably as simple as possible in view of manufacturing steps or material cutting, and for example, as shown in FIG. 5, it may be a cylindrical body cut with a specified length from a bar material.
However, when such a material is molded, since a corner portion 6 of the material shown in FIG. 6 is deformed at the beginning and fits into an upper mold 2 and a lower mold 3, this results in generation of a closed space 7. Once the closed space is generated, the space continues to exist until the completion of the molding, and a faulty lens is obtained because the molded material surface is not accurately conformed to the mold shape. Using FIG. 7, a conventional method preventing such a faulty product is explained. The lower mold 3 is fixed to a base 3b through a connecting rod 3a, and the upper mold 2 is fixed to a piston rod 2b through a connecting rod 2a. The material is heated to a molding temperature by a heater 8. At the time of attaining a specified molding temperature, the upper mold 2 is lowered by a piston 9 and contacts the material. After that, up-and-down oscillation pressing is carried out by the upper mold by, for example, using a servo-pulser 10. For example, 90% of a pressing stroke is covered by the oscillation pressing and the remaining 10% of the stroke is covered by a regular pressing. When total pressing stroke is completed, power application is stopped, and when the temperature is lowered to a specified temperature, the cavity is opened. Then, a lens is taken out after being cooled. It is disclosed that when 90% of the total pressing stroke is oscillation-pressed during the above consecutive molding processes, a non-contact portion conventionally generated is eliminated (for example, Japanese Patent Publication No. 60-246231). In this conventional molding method, however, there are problems that since close contact and separation are repeated between the upper mold determining a lens shape and a lens material during a molding step, that is, during a heating and pressing step, at that time, air is swallowed which generates bubbles in the softened material. In addition, owing to the behavior of the above upper mold, position matching with the lower mold is very difficult and guarantee of the inclination of both sides of a molded lens is very difficult. Besides, owing to behavior of the upper mold described above, since the temperature of the upper mold becomes non-uniform, temperature distribution of the lens material becomes non-uniform and a large deformation is generated in the formed lens.